Lark about with doctor in prison (6)
I believe the answer is:
gambol
'lark about' is the definition.
('gambol' can be a synonym of 'lark about')
'doctor in prison' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'in' is an insertion indicator.
'prison' becomes 'gaol' (I've seen this before).
'mb' placed inside 'gaol' is 'GAMBOL'.
'with' acts as a link.
(Other definitions for gambol that I've seen before include "Frisk and play" , "Skip" , "Spring" , "jump for joy" , "Frisk about and bet, by the sound of it" .)